1. Field of Invention
This invention relates to a device for nondestructive and noncontact detection of faults in a test piece by means of eddy current measurement or magnetic stray flux measurement having a stationary measurement means for taking an eddy current measurement or a magnetic stray flux measurement on a test piece which is continuously advanced relative to a measurement means; and a means for positioning the test piece with respect to the measurement means in a plane perpendicular to the direction of movement of the test piece. Furthermore the invention relates to a corresponding process.
2. Description of Related Art
A conventional measurement process for nondestructive and noncontact detection of faults in a test piece, especially a semi-finished metallic article, is induction measurement of eddy currents in the test piece. Here, the test piece is exposed to periodic alternating electromagnetic fields by means of a sinusoidally energized transmitting coil. The eddy currents induced thereby in the test piece, in turn, induce a periodic electrical signal in a coil arrangement used as a probe. The periodic electrical signal has a carrier oscillation according to the transmitter carrier frequency whose amplitude and/or phase is modulated by a fault in the test piece in a characteristic manner when the fault travels into the sensitive region of the probe. Conventionally, to scan the test piece, the latter is moved linearly with respect to the probe, but also arrangements with a rotating probe are known. One example of an eddy current measurement device with a linearly advanced test piece can be found in U.S. Pat. No. 5,175,498.
Another conventional measurement process for nondestructive and noncontact detection of faults in a test piece is known as magnetic stray flux measurement (or magnetic stray field measurement) in which, by means of an induction coil with a magnetic yoke, magnetization of the test piece is produced and the magnetic stray flux produced by the test piece is measured by means of a suitable sensor. Faults in the test piece are detected based on their effects on the magnetic stray flux. One example of such a stray flux measurement can be found in U.S. Pat. No. 4,445,088.
In order to be able to carry out an eddy current or stray flux measurement, the test piece must be supplied to the measurement device by means of a suitable guide means in a conventionally centered position which is exactly defined with respect to the measurement device, and this defined feed position is to be permanently maintained in operation. One example for this positioning means or guide means can be found in German Patent Application DE 198 22 986 A1 and corresponding U.S. Pat. No. 6,344,740 B1.
Since both in eddy current measurement and also in stray flux measurement the distance between the test piece and the measurement probe is relatively critical, it is important for the reliability and meaningfulness of the measurements to ensure the exact feed position of the test piece. Typically, the guide means for the test piece is not regularly monitored. Mainly, due to the rough environments in which eddy current and stray flux measurement devices are conventionally operated, for example, in steel mills, this can lead to the guide means for the test piece falling out of adjustment after a relatively short running time so that the reliability of the measurements can be a problem. In particular, the guide means is also exposed to the aging phenomena, for example, wear of the rolls. For a de-centered cylindrical test piece, this can lead to the same types of faults producing fault signals of different size depending on the position on the periphery of the test piece
In stray flux measurement, where dragging probes are used, de-centering of the test piece leads to increased wear and increases the danger that the probe heads will rise briefly or entirely; this leads to more or less long insensitivity of the test system. A differing sensitivity depending on the position of the fault on the periphery also occurs in stray flux measurement since the magnet yokes are moving on a fixed orbit around the test piece, by which the magnetization power is modulated with the position on the periphery so that, with de-centering, identical faults are unequally detected depending on the radial angular position.
For eddy current measurement devices with probes revolving around the periphery of the test piece, measuring the distance between the probe head and test piece is known in order to be able to correct the measurement with respect to the distance which fluctuates in the course of a revolution, for example, as a result of de-centering or asymmetry of the cross section of the test piece. One example of this arrangement can be found in German Patent Application DE 40 03 330 A1.
German Patent Application DE 44 21 912 A1 discloses a device for centering the spindles of textile machines in which two laser triangulation sensors which are located around the spindle at an angle to one another are used to determine the position of the spindles.
German Patent Application DE 101 14 961 A1 discloses a process for determining the outside contour and orientation of rolled steel test pieces moving in the lengthwise direction, distributed in the peripheral direction of the test piece there being several sensors for shadow measurements. A similar process is described in German Patent Application DE 10 2006 048 954 A1, a shadow measurement being combined with a split-beam measurement.